Note: Descriptions are shown in the official language in which they were submitted.
14266:C~:ia-73-6/17/82 -1-
DATA ENCODING AND SYNCHRONIZATION FOR
PULSE TELEMETRY
Field o the Invention
This invention relates to methods and means for encoding
and synchronizing data in pulse telemetry systems. The
invention is particularly suitable for use in processing
data acquired at the bottom of a well while the well is
being drilled.
The Prior Art
Data acquired at the bottom o~ a well while the well
is being drilled rnay be transmitted to the surface for
processing by pressure or acoustic signals transmitted
through the drilling fluid. U.S. Patents Nos. 2,924,432 to
Arps, et al; 3,789,355 to Patton; 3,949,354 to Claycomb;
3,964,556 to Gearhart, et al.; and 3,983,948 to Jeter
illustrate various arrangements for transmitting such data
to the surface through the drilling fluid. Usually the data
is encoded and transmitted in the form o~ either positive
or negative pressure pulses through the drilling fluid.
Noise signals produced by the drilling equipment and
drilling process tend tc, obscure some of the data pulses
in the prior art encoding arrangements, and when part of
the data pulses are lost it is difficult to determine the
particular parameters that are being measured due to the
.~
14266:CRH -2-
normal variation in the pulse transmission sequence.
In addition, the prior art encoding arrangements
require a substantial amount of time to restart logging the
well after a shutdown.
Summary of the Invention
In accordance with the present invention there is
provided apparatus for enc~din~ data in a pulse telemetry
system comprising means for producing a train of encoded
data pulses wherein the time intervals between pa;rs of
ld successive data pulses constitute measurements of the
magnitude of the data parameters, and means for producing
redundant pulses at known time locations with respect to
and between pairs of the data pulses, for enabling the
telemetry system to distinguish data pul.ses from noise
signals because of the known time relationship between
the redundant pulses and the encoded data pulses.
Also in accordance with the invention there is
provided apparatus for producing encoded data in a pulse
telemetry system comprising means for producing frames of
pulse encoded data having a known number of synchronization
intervals of equal time duration defined by synchronization
pulses at the beginning of each synchronization interval,
the pulse encoded data being arranged in data words which
constitute measurements of the magnitude of the data
parameters, means for producing redundant pulses in
preselected data words having known time positions with
respect to and between the pulses in the data words for
enabling the apparatus to distinguish data pulses from
noise, and mean~s for producing at least one identification
pattern of pulses at a known location in each frame, with
the pulses in each identification pattern being spaced
from one another by known intervals of time.
Further in accordance wi-th the ~n~ention there is
provided a ~ethod of well logging by telemetering
information in pressure pulses in the drilling fluid from
the interior of a well to the surface comprising producing
encoded data pulses wherein the time intervals between
- 2a --
pulses constitute measurements of the magnitude of data
measured in the well, producing redundant pulses a~ known
times with respect to and between the data pulses, and
producing pressure pulses in the drilling fluid in
accordance with the occurre,nce of ~he data pulses and the
redundant pulses.
Further in accordance with the invention there is
provided a method of wPll logg:ing by producing enoodad
data representing data parameters measured in a well
~hile drilling is being conducted comprising producing a
series of synchronization pulses spaced from one another
by a synchronization interval of known time duration,
producing encoded data pulses during each synchronization
interval wherein the time intervals between successive
data pulses constitute measurements of the magnitude of
the data parameters measured in the well, and producing
redundant pulses having a known time relationship with
respect to and between the data pulses to provide a pulse
sequence having a unique time relationship for distinguishing
data pulses from noise signals.
Further in accordance with the invention there is
provided a method of well logging by producing encoded
data representing data parameters measured in a well
while drilling is being conducted, comprising producing a
series of synchronization pulses wherein successive pulses
are spaced from one another by a synchronization interval
of known time duration, producing encoded data pulses
constituting data words during each synchronization
interval with the time intervals between successive pairs
of data pulses constituting measurements of the magnitude
of the data parameters, produc.ing said data words in
identified sub-frames of data in a known numerical
sequence, sequentially grouping a number of sub-frames
into framest with each frame representing one oomplete
3~ set of data and with the location of each data word being
at a known identifiable position in each frame, sequentially
2b
storing the sub-frame identifications, and applying the
sub frames o pulse data in sequence to a pulse generator
for producing pressure pulses for transmiSSion to the
surface of the well after a known time delay, whereby
when the well logging is shut down the last stored sub-
frame identification remains in storage and identifies
~he sub-frame number a~ which t!he well logging is to restart.
Further in accordance with the invention there is
provided a method of well logging by telemetering information
in pressure pulses in the drilling fluid from the interi.or
of a well to the surface comprising producing a series of
synchronization pulses wherein successive pulses are
spaced from one another by a synchronization interval of
known time duration to define sub-frames of information,
producing data words in known sequences during each
synchronization interval in the form of data pulses
wherein the time intervals between known pairs of pulses
constitute measurements of the magnitude of data parameters
measured in the well, with some preselected data words
providing identification patterns of pulses with the
pulses in each pattern being spaced from one another by
known intervals of ~ime, producing redundant pulses having
known time relationships between selected data pulses for
distinguishing data pulses from noise signals, sequentially
combining a known number of sub-frames into frames, with
each frame constituting one complete set of data,
sequentially storing the sub-frame numbers and the time
at which they are stored, and producing pressure pulses
after a known time delay in accordance with the successive
sub-frames of data pulses, so that the last stored sub-
frame number remains in storage and identifies the sub-
frame number at which the well logging method should
restart after a shutdown.
- 2c
The aforesaid difficulties are overcome in the present
invention by the use of a pulse code wherein the time
intervals between successive pulses in a pulse train
are represenative of the magnitude of the data parameters
and provide measures of the data parameters, and by including
redundant pulses at predetermined locations in the series
of encoded data pulses so as to enhance the ability to
recognize or distinguish data pulses over noise signals.
The pattern of the redundant pulses with respect to the
data pulses is such that they have a low probability of
being generated by random noise. In addition, synchroni
zation pulses may be employed at the beginning of each
predefined unit of data information to enable the identi-
fication of the particular parameters being telemetered to
the surface even though one or more of the pulse code
signals that provide a measure of a parameter is obscured
by noise.
The sequence of the production of the pulse code
signals is continuously monitored to enable the tele-
.~ metering system to be activated promptly after a shutdownat the place in the train of data where it would be if
the shutdown had not occurred.
The encoding, synchronization and sequencing can be
provided by programming a microprocessor or by hardware.
1~266:CRH -3-
1 Brief ~escription of the Dra_ n~
Fig~ 1 illus-trates one form of the pulse code employing
redundant pulses;
Fig. 2-4 illustrate alternate pulse codes employing
redundant pulses,
Fig. 5 illustrates three types of identification
patterns that may be incorp~rated into the pulse codes;
Fig. 6 shows how the pulse code signals may be produced
by circuit hardware and employed to produce negative pressure
pulses in drilling fluid;
Figs. 7 and 8 illustrate how the analog parameters may
be converted to binary signals under the control of a micro-
processor so as to produce the pulse codes;
Fig. 9 illustrates how a synchronized restart may be
incorporated into the encoding scheme so that the system
may be activated promptly after a shutdown at the p~ace in
the train of data where it would be if the shutdown had
not occurred; and
Fig. 10 illustrates how the pressure pulse signals may
be processed at the surface to provide a read-out of the
parameters that are measured down hole.
'(9 ~l.t' I
14266:CRH -~-
1 Description of the Preferred ~mbodiments
. . . _ . .
A pulse may be defined as a predefined sequence of
changes of state (e.g., pressure, voltage) within a fixed
period of time. In practice, a pulse may be a drop in
5 pressure followed by a return [increase] to normal pressure,
with normal pressure being the pressure within the circu-
lating system for the drilling fluid without the pulse.
Negative pressure pulses are preferred, but the encoding
scheme is equally applicable to positive pressure pulses.
In ~he illustrations herein the duration of a pulse is
considered to be a drop in pressure for a fixed period of
time (e.g. 1 second or 0.5 second) followed by one second
of normal pressure.
The measurement of the various parameters in the well,
such as gamma ray, forma~ion resistivity, magnetometer,
temperature etc., is in analog form. The time intervals
between successive data pulses in a pulse train provides
the analog measurement of the respective parameters.
A "frame" is defined to be the period in time required
to transmit one complete set of data. In order to maintain
synchronization within the frame, the data is ordered into
"sub-frames"O Each sub-frame is one "synchronization
interval" long, and the start position is marked by a
synchronization pulse.
Table 1 illustrates a scheme which employs 15 sub-
frames. Two "DATA WORDS" are transmitted during each
sub-frame except for sub-frame 15 where one data word is
transmitted along with a test pattern and "FRAME IDENTIFI-
CATION" pulses. A data word is deined to be the equivalent
of an 8-bit binary number (i.e. an integer in the range 0
to 255). The frame identification pulses (or "frame synch-
ronization" pulses) enable identification of the first
sub-frame, and hence, enable the pulse detector/decoder
at the surface to identify the sequence of data transmission.
14266:CRH -5-
1 TABLE 1
Sub-Frame Data Word #1 Data ~lord #2
1 Gamma Ray Time of Day
2 Gamma Ray Mud Temperature
3 Gamma Ray Mud Resistivity
4 Gamma Ray Battery 1
Gamma Ray Battery 2
6 Gamma Ray Battery 3
7 Gamma Ray Battery 4
8 Gamma Ray Pressure 1
~ Gamma Ray Pressure 2
Gamma Ray Pulse Pressure
11 Gamma Ray Pressure Variation
12 Gamma Ray Borehole Inclination
13 Gamma Ray Hydrostatic Pressure 1
14 Gamma Ray Hydrostatic Pressure 2
Gamma Ray Test ~ Frame Identi-
fication Patterns
~5
1~26~:C~ 6-
1 Table 2 illustrates a sch~ne which employs 11 sub-fra~es and three
data words in each sub-frame.
TABLE 2
Sub-E'rame # Data 1 Data 2 Data 3
. .. _ . ~. ~
Conventional Data
1 Gamma Ray Formation Resistivity Frame Ident Pattern
2 Gamma Ray E'ormation Resistivity Formation Resistivity
3 G~nma Ray Formation Resistivity Mud Resistivity
4 Gamma Ra~v Formation Resistivity Formation Resistivity
Gamma Ray Fon~ation Resistivity Mud TemFerature
6 Gamma Ray Formation Resistivitv Formation Resistivity
Directional Data
.. .
1 Survey Temp Survey Accuracy Frame Ident Pattern
2 Accelercmeter X Accelerome-ter Y Accelerc~eter X
3 Accelerometer Y Magnetcmeter X Magnetometer Y
4 Magnetometer Z Mag X ~ Mag Z May Y + Mag Z
Test Pattern
1 Space available for test pattern Frame Ident Pattern
3~'~
1~266:CRH -7-
1 When the ]ast sub-frame is completed, tr~nsmission
resumes at sub-frame 1 with no delay, with the sync pulse
for sub-frame 1 being one synchronization interval after
the sync pulse for the last sub-frame (i.e. sub-frame 15 in
Table 1).
After synchronization has been attained (the sync
pulses are recognized as the only pulses invariant over one
sync interval and the sub frame numbers can be decoded
after the frame identification pulses are recoynized), the
surface detector/decoder can assume (interpolate) the
position of the following sync and identification pulses
and only the data pulses need to be detected in order to
correctly decode all data. Synchronization is maintained
during periods where no data pulses are transmitted, and
hence it is possible to restart decoding data immediately
after a connection without any resynchroni~ation delays.
In practice, the surface detector/decoder continuously
attempts to search for sync and identification pulses to
accommodate any small drift in the timing clocks of the
2~ downhole and surface equipment.
A sub-frame has been defined as an interval in time of
duration one sync interval started by a sync pulse. In
order to transmit two data words per sync interval, two
additional pulses are required. The time from the end of
the sync pulse to the first (data) pulse is proportional to
the data word (#l) being transmitted
Time 1 = Data 1 * dT
Where dT = The time for one unit o transmitted data
Data 1 = Number of transmission units (Data value in range
0-255)
In a similar fashion, the time from the end of the first
data pulse to the second data pulse is proportional to the
second data word (#2).
14266:CRH -8-
1 The information necessary to discriminate between data
pulses and noise pulses is supplied by generating a pattern
of pulses which have a low probability of being generated
by random noise. This requires additional pulses to create
the patterns.
Fig. 1 illustrates a pulse scherne which is suitable
for transmission of the data in Table 1. Each sub-frame has
two data words, and the analog measures of the data parameters
comprises the time between the sync pulse and Dl and the
time between Dl and D2.
A "redundant pulse" R is provided midway between adjacent
pairs of data pulses~ It is employed to discriminate between
time data pulses and noise pulses. Thus, any two data
pulses can be considered to be data pulses (Dl and D2) only
if there exists one additional pulse (R) midway in time
between the data pulses.
The timing of the scheme of Fig, 1 may be as follows:
The duration of a pulse is two seconds. (A one-second
drop in pressure followed by one second of normal
pressure.)
One Unit of Time (dT) = 3/32 seconds
Full-Scale Data ~Jord = 255 * 3/32 = 23.9 seconds
Maximum Data Transmission Time = 2 full-scale data words
+ 4 pulses
= 55.8 seconds
Sync Interval = 60 seconds
(The sync interval could have been selected as 55.8 seconds.
Sixty seconds was chosen as a convenient interval for testing.)
14266:CRH ~9~
1 Although the encoding scheme deals with the equivalent
of 8 bit (binary) numbers, this does not limit the system
to the transmission of two "pieces of information" per sub-
frame. ~en the (analog) number is decoded on-surface,
it i5 a simple matter, using a digital computer, to recreate
the 8 indlvidual (binary) bits of information that were
~ransmit~ed by the downhole Tool. Thu~ we are, in essence,
transmitting 16 independent pileces of info~ation each sub-
frame (2 8-bit words~. ~
This concept is us'ed in t:he transmission of the pressure
informa~ion ln sub-frames 8/9 and 13/14 of Table 1. The
downhole sensors are measured using a 12-bit ADC. Althoush
the additional accuracy obtainable using a 12-bit number
tl/4096 compared to 1/256 for an 8-bit number) is not
necessary for all sensors, it is necessary to resolve
bottomhole pressures (several thousand psi) to a few psi.
The measured 12-bit number was transmitted as two 8 bit
numbers with a 4-bit overlap as shown below:
All transmitted mud pulse data is represented (downhole~
as eight bit binary numbers.
M5a ~53
2S e.g. 102 d~i~l 8 Cl I ~ <~ U5~ ~or G~ ~Lt
8 bit~ ~ 0-255 bit 7 i 4 ~ D R13
~J bit-a ~:e ~e to
Lna~acv irl d~
A 12 bitblt ~ 7 ~ 2 . c
~r Qn il3 a ~ 0 1 0 ¦ 0 ~ < ~ f~ D~15CN~
b~ S~VI~Y DA3
~0 lap~t a bit .
- 1~1 3 l-a~
12 bit~ ~ 0~095 Af~ cc~ini~ d2lta, er~or
14266:CRH -10-
1 "Press~re 1" is the most significant 8 bits of the
raw 12-bit data and "Pressure 2" is the least significant 8
bits of the raw 12-blt data. Since the data words are
transmitted using an analog scheme, the probability of
correctly decoding the individual data bits is greatest for
the most significant bits. (If there i5 an error in decodin~
due to the inability to resolve the data pulses to the
required interval [approximately Ool second], then a small
error will be introduced- the number 211 may be decoded as
212, for example.)
Fig. 2 illustrates a pulse scheme which is suitable for
transmission of the data in Table 2.
The duration of one pulse is reduced to a half-second
drop in pressure followed by one second at normal pressure.
The time for one unit of data (dT) was reduced to 1/16
second. Instead of reducing the sync interval, a third (8-
bit) data word is transmitted in the interval following the
second data word. One additional redundant pulse was required
between the second and third data pulses to ensure
identification of the third data pulse.
The timing is as follows:
One pulse = 1.5 seconds
(0.5 sec drop + 1 sec normal)
One Unit of Time (dT) = 1/16 seconds
One Full-Scale Data Word = 255 * 1/16 = 15.9 seconds
Maximum Data = 3 full-scale data words + 6 pulses
Transmission Time = 56.8 seconds
Sync Interval = 60 seconds
14266:CR~
1 Fig. 3 lllustrates a pulse scheme which is similar to
that of Fig. 2, with the redundant pulses Rl midway in time
between the adjacent data pulses Dl and D2, but with
redundant pulses R~ not midway in time between the adjacent
data pulses D2 and D3. Ratherl the pulses R2 can be spaced
any desired time ratio between the adjacent data pulses.
Such an encoding scheme is asymetrical in time and is
advantageous when multiple "groups of three" (2 data + 1
redundant) pulses appear within one sub-frame. If desired,
both of the redundant pulses Rl and R2 may be asymmetrical
in time with respect to the adjacent data pulses.
Although the encoding scheme of Fig. 2 has been selected
to fi~ three full-scale data words per sub~frame, the data
is normally not full-scale and there is therefore "unused"
time at the end of most sub-frames during which additional
data can be transmitted. The encoding scheme can be set up
to transmit additional data if time is available. This
will change the data rate from the current fixed value of
three (8-bit) words per minute to an average data rate
which will not be less than the current rate.
Although the encoding scheme of Fig. 2 requires two
pulses (e.y. R2 and D3) for the addition of one data word,
it is possible to use a redundant pulse after every second
data pulse or between alternate pairs of data pulses and
maintain a check on all data.
Fig. 4 illustrates such an encoding scheme which in-
creases the total number of words transmitted within a
sub-frame by the use of a redundant pulse between alternate
pairs of data pulses.
The maximum data rate is limited by the pulse width.
Consider the following example:
Pulse Width = 1.5 seconds
Sync Interval = 60 seconds
A maximum of 40 pulses could be transmitted.
~ ~3, ~ 7
14266:CRH ~12-
1 At three pulses per pair of data words, this could be
interpreted as 13 pairs of (zero valued) data words, i.e.,
26 data words per minute.
Another example considers average data values. ~or
8-bit data words, the full-scale data value is 255, the
average is therefore 127.5. At 1/16 seconds per data unit
this corresponds to 8 seconds per data word.
Total Time = Sync Pulse Time + n*(8 sec + 1.5 pulses data word
60 seconds = 1.5 seconds + n*(9.5 seconds)
therefore, n = 6.2 where n is the number of data words
Although a downhole Tool is not expected to produce
random data, most sensors (gamma ray and formation resisti~ity)
are scaled such that common sensor readings are approximately
mid range to accommodate unusually high measurements as
well as unusually low measurements. Hence, data will tend
to supply average valued data in "typical" formations.
As illustrated in Fig. 5, the frame structure may be
extended to accommodate different data types. Three
different frames may be transmitted. Conventional Frames,
Directional Frames and Test Pattern Frames. Each frame has
a predetermined number of sub-frames. The frame ident
patterns are transmitted at the end of the first sub-frame
of each frame.
With the pulse encoding scheme of Fig. 2, whenever a
frame identiication pattern is transmitted, only two data
words are included in the sub-frame to ensure that there
is no overlap between the frame ident pattern and the
third data pulse. Part of the frame ident pattern is
transmitted in the period between the full-scale data time
of 56.8 seconds and the end of the sub-frame. This helps
with the identification of the frame pattern since data
pulses are never transmitted in this period.
14266:CRH -13-
1 The programming schemes of Figs. 1-5 may~be produced
by programming a suitable microprocessor, such as the RCA
1802, or by circuit hardware.
Fig. 6 illustrates a way of producing the encoding
schemes with electric circuit hardware. The illustration is
directed to the pulse scheme of Fig. 2, and it will be
apparent that the circuit may be modified to produce any
of the pulse schemes that have been discussed above.
The measurement of only three data parameters for one sub-
frame is illustrated for simplicity. It will be apparent
that additional data parameters may be measured in the
system for the selected number of sub-frames.
The data to be transmitted to the surface is measured
in analog form, such as voltage, by the sensors 20, 22, and
24. The voltages that are produced by sensors 22 and 24
are divided by dividers 26, 28, 30 and 32 for use in
producing the redundant pulses Rl and R20
The voltages so produced are scanned by a stepping
switch 34 which is actuated by a coil 36. A terminal 38 of
the switch is qrounded and it represents time zero in each
sub-frame. A terminal 40 of the switch is connected to a
battery 42 which provides the synchronizing pulses.
The rotor of the s~epping switch 34 is connected to
a resistor-condenser network 44, 46 which is connected to
a trigger circuit 48. The time constant of the resistor-
condenser network 44, 46 causes the trigger to producepulses of 0.5 second duration at times that are represen-
tative of the voltages produced by the respective sensors~
Thus, the time spacing between the data pulses that are
produced at the output of the trigger 48 is an analog
representation of the magnitude of the data as measured by
the sensors 20, 22 and 24.
14266:CRH -14-
1 If the redundant pulses ~1 and P~2 are to be equally
spaced in time between the adjacent data pulses, the dividers
26, 28, 30 and 32 divide the voltages that are produced by
the sensors 22 and 24 by two.
S The rotor of the stepping switch 34 is on terminal 38
at time zero. Upon actuation, the rotor moves to terminal
40 which carries a voltage which immediately actuates
the trigger 48 to produce a sync pulse. Upon further
actuation, the rotor scans terminal 50 and the trigger 48
produces data pulse Dl. Thereafter it engages terminal 52
and the trigger 48 produces redundant pulse Rl. Next it
enga~es terminal 54 to ~roduce data pulse D2. Redundant
pulse R2 and data pulse D2 are produced when the rotor
scans terminals 56 and 58.
The coil 36 of the stepping switch is first energized
by a pulse produced by the oscillator 60 and pulse generator
62 which produce one pulse each sixty seconds which are applied
through an amplifier 64 to the coil 36. Thereafter during
one sync interval the coil 36 is energized in steps by the
pulses that are produced by the trigger 48.
A counter 66 is connected to the oscillator 60 and the
pulse generator 62. It serves to activate data frame
identification pattern generator 68, directional frame
identification pattern generator 70, and test frame identi-
~5 fication pattern generator 72 at the times that thoseframes are to be inserted into the pulse scheme of Table 2.
The data pulses and redundant pulses and the frame
identification patterns are applied through an amplifier 73
to a valve 74 to cause it to open and produce negative
pressure pulses in the drilling fluid in response to each
of the pulses.
Various types of valves 74 may be employed. A suitable
valve 74 is disclosed in Canadian Paten-t No: 1,162,143
issued 14th February 1984.
14266:CRH -15-
1 A suitable control circui-t for actuating such a valve is
disclosed in U.S. Patent No: 4,336,564 issued on 22nd June,
1982.
A valve for producing pos.itive pu:Lses is disclosed by
application Serial No: 421,768 filed 16th February, 1983~
Figs. 7 and 8 show how ~he analog pulses may be produced
under the control of a microprocessor, suc~ as the RCA 1802.
The order of transmission o~ the differen~ frame types
is variable and is set up within the microprocessor which
operates the downhole Tool. The sequence is best shown by a
flow diagram:
t ~OOL ResrAa~ ) Restart a~t~r power failure
l ~
- < : Directional Survey Frame Enabled ?
r Directional Setpo~nts Satisfied ?
~. ~ ~ Start Directional Survey (Frame)
_ . ~ ConYentional Data Enabled?
-~ Transmit Test Pattern (Frame)
r-~-- ~ Restart ConYentional Frame in Sequence
~ _~ Start Conventional Frame
Continue with Conventiona1 Frames until Power Fails
14266:CRH ~16-
1 Fig. 7 is a simplified block diayram showing the method
of settiny up pulse timing in response to the encoding
scheme of Fig. 2. Three data words are transmitted in one
synchronization interval~ The synchrorli~ation interval is
60 seconds.
When the Tool restarts it measures data for 59 seconds.
In the following second the three data words (8-bit numbers
= integers in the range 0-255) are calculated and the
pulse table is set up for the next minute.
The pulse table is a shift register 76 (960 bits long)
which is clocked out at a rate of 16 bits per second. It
takes exactly one synchronization interval (60 seconds) to
clock out all data.
The data from the shift register 76 controls the pulsing
valve 74. While a "one" is present at the output from the
shift register the pulsing valve 74 will remain open and a
pulse will be transmitted.
During the first minute of Tool operation after a
restart there is no data to transmit. However, after the
first minute, data is transmitted (clocked out of the shift
register) at the same time that new data is being measured.
The sequence to SET UP PULSE TABLE involves setting a
string of "ones" into the Pulse Table Shift Register 76 to
transmit a pulse, and "zeroes" to mark the time between
pulses, as shown in Fig. 8. Since the bits are clocked out
of the shift register at the rate of 16 per second, a
sequence of 8 "ones" is required to generate a one-half
second pulse.
3~'7
14266:CRH -17-
1 Since there are no redundant pulses between the sync
pulse and the first data pulse the number of zeroes required
for Datal is equal to the value of the data (0-255). The
remaining two data words are "split in half" by the redundant
pulses. In the case of Data2, the number of "zeroes"
corresponding the first half, (Data2)/2 rounded down to
the nearest integer, is set up prior to the first redundant
pulseO The second half, (Data:2~1)/2 rounded down to the
nearest integer, is set up after the redundant pulse, before
the second data pulse.
The extra "~1" is required in the second "half" since
2 does not divide an odd number exactly. Consider the two
examples:
Transmitted Data = 122,
(Data~/2 - 61.0, (Data+l)/2 = 6105 rounded to 61
Decoded data = 61~61 - 122.
Transmitted Data - 123
(Data)/2 = 61.5 rounded to 61.0, (Data+l)/2 = 62
Decoded data = S1~62 - 123.
A similar procedure is used to set up Data 3.
Fig. 9 shows the changes to the scheme that are required
to ensure that data starts up in synchronization with the
data transmitted before the restart.
Two pieces of hardware are required to enable the
synchronized restart. The irst is a battery backed up
clock. This enables the Tool to "wait" until a new
synchroni.zation interval starts (once per minute, every
14266:CRH -18-
1 time "seconds" reads zero) before restarting the
measurement/transmit sequence~ The second is a battery
backed up memory register. Every time new pulse data is
set into the pulse table shift register the time and sequence
S number (sub-frame number) are stored in battery backed up
memory registers. If the Tool is shut down and then
restarted this data is saved and can be inspected. The
Tool can therefore restart at the point in its predefined
sequence where it would have bleen if it had not shut down.
The predefined sequence (or Frame) defines which data
to measure and transmi~ - different sensors may be measured
each minute (each synchronization interval~.
The surface (decoding) equipment always assumes that
the downhole Tool transmits its data in the order of the
predefined sequence and therefore does not need to re-
synchronize with the downhole Tool every time the downhole
Tool restarts (every connection).
Fig. 10 illustrates one arrangement for processing the
encoded data at the surface.
The drilling rig includes the usual rotary table 90,
kelly 92, swivel 94, traveling block 95, mud pumps 96, mud
pit 98, and a drill string made up of drill pipe sections
100 secured to the lower end of the kelly 92 and to the upper
end of a drill collar 102 and terminating in the drill bit
2S 104. The down hole pulse encoding apparatus and the valve
74 for producing negative pressure pulses may be located in
a drill collar 106 located above the drill bit 104.
A pressure transducer 108 is coupled to the conduit
for the drilli.ng fluid and it senses the negative pressure
pulses that are produced down hole.
The mud pumps 96 produce noise signals that have
certain characteristics. A strobe generator 110 produces
strobe signals for each cycle of each mud pump.
14266:CRH -19-
1 The signals from the pressure transducer,108 and ~he
strobe generator 110 are applied to an input module 112
where the pressure signals are averaged in pairs and the
average is stored 50 times per second in a buffer until
required by the filter module 114.
The filter module 114 serves to remove or reduce the
noise signals produced by the mud pumps 96. Various types
of such filters are ~nown in the art. ~owever, the filter
is preferably of the type shown inAcopénding appli~ation
Serial No. ~` ~ 7 ~ o 1
.
The output of the filter 114 is applied to a detector
116 which employs a matched filter to enhance the detectability
of the encoded pulses.
The pulse signature used in the matched filter is a
15 firs order approximation to a rectangular pulse which has
been high-pass flltered at a frequency corresponding
to l/(4*pulse width) hz. The use o a high-pass filter
allows a simple level detector to be used for pulse
identification.
pss
"Ideal" mud pulse:
5
t - pulse width
Actual matched filter ~ ~ ~
pulse signature: U o
/~ /\ P5~: 5eco"DS
Output from matched f~lter:
The sharp peak coresponds ~v
to the best match between
actual pulse and filter
signature.
14266:C~H -20-
1 A high speed FFT convolution technique is used to
implement the matched filter convolution. In operation,
the convolution is performed approximately once every 20
seconds whenever data is made available by the filter module 114.
The output of the matched filter is stored in a "Detection
Buffer" which can hold data for more than one complete sub-
frame. (Since the current sub-frame is 60 seconds long,
the buffer is set up to hold data for a 64-second interval.)
A simple level detection scheme is performed on the
lQ (overlapping) 64-second intervals. (The detection level
is a user-contrvlled parameter.) Pulse DoSitiOn ( time) is
assigned at the pulse minimum; pulse height and width are
also determined. The decoded pulses are stored in a "Pulse
Table" for use by the decoder module 118. If pulse width
lS does not fit within certain predefined limits, then the
pulse is considered to be "noise" and discarded.
Sub-frame sync pulses are found by a search of invariant
pulses of one-minute periods over the last three minutes.
When these pulses are found, the detection buffer is shifted
in time to ensure that the start of the sub-frame will be
located at the start of the buffer. The time of the sync
pulse is recorded for use by the decoder module 118.
Since three minutes are required for the identification
of sync pulses, the first time a downhole Tool starts up
downhole, a search for frame ident pulses is enabled within
the detector module 116 before the sub-frame sync pulses
are identified. This enables the apparatus to synchronize
on the first sub-frame that is transmitted from downhole.
3S
14266:CRH -21-
1 The output of the detector 116 is applied to a decoder
118 which identifies the pulses and calculates the values.
This module operates once every sub-frame. Every time
the detection buffer is filled, the decoder module 118 uses the
information within the Pulse Table set-up by the detector
module 116 to identify pulse types.
Sync pulses are first identified by their position in
time. Frame ident pulses are then searched for, again at
predefined time positions~ All pulses are allowed a small
error in time setup as a user setpoint called "Pulse Position
Variance" ttypically 0.1 second). All remaining pulses are
searched to identify the "groups of three equally spaced
pulses" which identiy data and redundant pulses.
Once the pulses are identified sub-frame numbers and
data values can be calculated and supplied to the output
buffer 120 via the Decoder Buffer.
The output buffer 120 transmits the data within the
decoder 118 to a computer 122 or a printer 124 for setting
forth the data parameters that are transmitted to the surface.
~,
